1. Field of the Invention
This invention relates to the determination of the concentration of ions in solution and, more particularly, to such determination by potentiometric means.
The potentiometric determination of the electrochemical activity of ions in solution is a technique which originated almost a century ago. The first practical application was that of the measurement of hydrogen ions or pH as a measure of acidity, pH being the negative logarithm of hydrogen ion activity. Such an application is still the most widespread one, although more recently several other cations (e.g. sodium, potassium, calcium) and some anions (fluoride, chloride) are also routinely measured today by this technique.
In all of the above noted applications, two electrodes are immersed into the solution to be tested and they are connected to a high impedance voltmeter. The latter measures the potential developed by the working electrode which is selectively sensitive to the ion to be measured. This working electrode potential is measured against the potential of the reference electrode, which is a constant potential. Effectively, then what is measured is the potential difference developed by a voltaic cell, which is a function of the pH of the solution being tested. Accordingly, the pH can be determined to a high degree of accuracy if the potential difference of the cell can be measured accurately.
As to the working electrode, per se, its most important element is an ion-sensitive membrane which, in the case of pH measurement, is a glass of specific composition (see, for example, M. Cremer, Z. Biol., 47, 562 (1906); F. Haber and Z. Klemensiewicz, Z. Phys. Chem.67, 385 (1909). Because it is impractical to make an electrical connection directly to the inner glass surface, an auxiliary electrochemical cell is set up and connected to the potentiometer. Such cell conventionally consists of a silver wire coated with silver chloride. The working electrode is then filled with a solution which has an ion concentration close to the central concentration value of the ions to be measured, and which also contains chloride ions to set up a potential with the silver chloride. The working electrode, therefore, conforms to the Nernst equation: ##EQU1## where E is the potential measured, R is the universal gas constant, F is the Faraday constant: 96,500 coulombs, T is temperature, n is the valence of the ion, E.sub.o is the standard potential, and a.sub.s the activity of a standard or reference solution, and a is the activity of the ion to be measured.
In practice then, the two electrodes are immersed in a standard solution of known activity or concentration, and the instrument is set to read this value. On the other hand, when the electrodes are immersed into an unknown solution, the potential E is a direct measure of the unknown activity according to this logarithmic relationship. The only other concern is the slope, which changes with temperature and may also fall short of the theoretical "Nernstian" behavior. Consequently, in practice, potentiometric instruments are provided with a "standardizing control" which provides a set point, and a "calibration control" which allows a change in slope. However, because the linearity of the measurements is close to perfect, a one or two point calibration suffices.
The reference electrode is similar to the inner part of the working electrode in that it contains, typically, a silver or silver chloride element surrounded by a solution containing chloride ions. Thus, this electrode has a constant potential. Another requirement for the pH measuring device is that the reference electrode must be electrolytically connected to the solution to be tested, that is, either the unknown sample or a standardizing solution, but without significant mixing of the inner contents of the reference electrode and the sample solution. This is usually achieved by means of a porous plug acting as a so called "electrochemical junction".
Over the last fifty years, a very large variety of instruments have been designed and produced to measure ion concentration and particularly to measure pH. Almost all of these instruments follow the same basic principles and the variations therein mostly relate to convenience and ease of manufacturing, or to new transducers responding to various ions not measured theretofore.
For special background material which will place the present invention in proper context, reference may be made to U.S. Pat. No. 2,058,761 and to U.S. Pat. No. 3,753,084. In the former patent, which dates to the 1930's, an apparatus is described for testing acidity whose main feature apparently relates to the provision of a potential indicating device of extreme sensitivity resulting from the ability to measure the potential of a high resistance circuit of a voltaic cell in an electrochemical environment. In such apparatus, one of the electrodes consists of a thin-wall glass bulb containing a solution of hydrochloric acid and quinhydrone of known characteristics; whereas the other electrode consists of a tube containing a solution of potassium chloride and mercurous chloride (calomel), the construction of that tube being such as to permit contact between the solution of potassium chloride and calomel within the tube and the solution to be tested. U.S. Pat. No. 2,058,761 proposes the substitution for a delicate galvanometer, previously considered necessary, a simple and mechanically rugged milliameter in combination with a specially designed vacuum tube amplifier.
The latter patent noted above, that is, Wirz U.S. Pat. No. 3,753,084 discloses an apparatus for the rapid electrometric determination of ionic activities, especially the pH value. According to the invention described therein, there is provided as the sensor an electrode measuring chain possessing measuring and reference electrode means. One of the principal features described therein is a storage vessel which is provided for the measuring chain, the storage vessel containing an activation solution to maintain said measuring chain under the influence of the solution when the measuring chain is not being employed for the purpose of obtaining the ion concentration of an unknown. The activation solution mentioned in that patent can have the same chemical composition as the conductive electrolyte in which the reference electrode wire is immersed, such conductive electrolyte containing stabilization additives and the like.
In any event, regardless of the particular merits of the devices already developed in the prior art and the particular apparatus described in the specific references cited above, none of the previous developments make it possible to take full advantage of state-of-the-art electronics and to avoid difficulties involved in periodically adjusting the instrument, i.e. of utilizing standardizing and calibration controls so that assurance is gained of accurate readings of particular electrochemical activities for given ion concentrations.
Accordingly, it is a primary object of the present invention to eliminate the need for an operator to carry at least one standardizing solution with a pH measuring device so as to enable him to periodically adjust the device to take care of variations in set point which is bound to occur with use.
A further object of the present invention is to provide a truly portable potentiometric instrument which is self-contained and no larger than a typical marking pen. The practical importance of such a device is great because a large proportion of important measurements of this character must be made by semi-skilled operators in locations removed from a chemical laboratory. The present device allows this to be done in a manner akin to temperature measurements with a simple thermometer.
The above noted primary object of the invention is fulfilled by the inclusion of a reservoir of standardizing solution as part of the combination of two electrodes which are essential to the operation of the measuring device. In other words, rather than carrying a separate container of standardizing solution, the container or reservoir of such solution is built into the measuring device. Preferably, the way this is done is to have the reservoir solution correspond with the reference solution. Thus the reference solution can consist of the requisite chloride, such as sodium chloride, along with buffering agents as required; furthermore, a means is provided at the lower end of the housing for the two electrodes so as to be able selectively to permit fluid to flow from the reservoir into the standardizing chamber so as to surround the lower end of the working electrode when standardization is desired. This means preferably takes the form of a valve, which also serves as the electrochemical junction of the reference electrode in the preferred form.
Accordingly, broadly stated, the invention resides in an ion measuring device adapted to store and discharge standardizing solution comprising a housing; a working electrode, including an electrode wire immersed in a solution contained within a sealed, ion-selective envelope, disposed in said housing; a storage reservoir adjacent said envelope containing standardizing solution; a reference electrode, including an electrochemical junction and an electrode wire immersed in a reference solution, disposed in said housing; a standardizing chamber defined at the lower end of said housing and surrounding at least a portion of said working electrode; means at the lower end of said housing for selectively providing communication between said standardizing chamber and said reservoir, such that fluid flows from said reservoir into said standardizing chamber, thereby to wet the ion-sensitive surface of said working electrode and to wet the reference electrode electrochemical junction when said communication means is activated for standardization purposes.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been given like numbers.